Pressing techniques for making shaped glass articles generally fall into one of two categories. In the first category, molten glass is used as a starting material. The molten glass, in the form of a gob, is pressed into a desired shape with a plunger. Where the molten glass has to be spread thinly to make a thin-walled glass article having complex curvatures, the molten glass may become cold, or form a cold skin, before reaching the final desired shape. Shaped glass articles formed from pressing a gob of molten glass may exhibit one or more of shear marking, warping, optical distortion due to low surface quality, and overall low dimensional precision. In the second category, preformed glass is used as a starting material. The preformed glass is reheated to a low viscosity and then pressed into a shaped article. This disclosure relates to the second category of pressing techniques.
U.S. Pat. No. 5,368,625 (Mizusugi) and U.S. Pat. No. 6,158,247 (Didelot) describe pressing techniques according to the second category. In the Mizusugi patent, a preformed glass is made into a preliminary bent shape. The bent preformed glass is then transferred to a press, where it is pressed into a desired shape between an upper die and a lower die. In the Didelot patent, a stack of two glass sheets is formed, with a separating agent interposed between the glass sheets. The glass sheets are heated and sagged by gravity onto a frame. A shaping part is brought into contact with the glass sheets to shape a central region of the glass sheets. The frame includes cross members, which are pivoted to bring an edge region of the glass sheets into contact with the sides of the shaping part, thereby forming the shaped article.